This invention relates to voltage measuring devices and in particular to a high voltage probe capable of measuring short duration high voltage pulses in an environment where high levels of electromagnetic noise are present.
Devices of this type find particular utility in applications where it is necessary to measure very fast high voltage pulses in a very noisy environment where a common reference potential (ground) is not clearly defined. One example of this type of application is a probe that measures high voltage transients developed when a metallic foil fuses while carrying very large currents.
State-of-the-art methods and devices for measuring the voltage across a fuse in this type of application have generally proved to be less than satisfactory. Many different techniques have been tried. These techniques included resistive dividers, capacitive dividers and capacitive V-dot probes. None of these proved to be wholly satisfactory. All of the foregoing approaches attempted to measure a known fraction of the fuse voltage directly. However, the fuse commonly is separated from the nominal ground by significant inductances (.about.20 nH). Moreover, rather large dI/dt's (.gtorsim.1 ma/us) are produced during the course of experiments that measure the voltage at the time the foil is fused. Consequently, a large amount of electrical noise is produced which swamps the signals from the probe.
The several limitations of prior art type voltage measuring devices can be overcome by a new alternative approach that comprehends measuring the current flowing through a known resistance connected across the fuse being tested. However, in order for this type measurement to be accurate, two conditions must be met. First, the shunt capacitance must be small enough that the capacitive resistance is much higher than the resistance at the frequencies of interest. Second, the inductive reactance in the resistor must be much smaller than the resistance at these frequencies. Conventional resistances, however, fail to meet either of these criteria.
In accordance with the foregoing it is apparent that there currently exists the need for a high voltage probe that overcomes the limitation of prior art voltage measuring devices. The present invention satisfies that need by providing a voltage measuring device based on the concept of measuring the current flow through a known resistance. The present invention also utilizes a novel resistance element that overcomes the above noted limitations of conventional resistance elements.